basic aerodynamics ii stability large

04/18/23 Author: Harry L. Whitehead 1

Basic Aerodynamics

III. Basic Aerodynamics A. The AtmosphereB. PhysicsC. The AirfoilD. Lift & DragE. StabilityF. Large Aircraft Flight

ControlsAircraft Stability•Stability: General

•To insure an airplane has good handling qualities in all flight regimes, we need it to be STABLE, MANEUVERABLE, and CONTROLLABLE•STABILITY is the characteristic of an airplane in flight that causes it to return to a condition of equilibrium, or steady flight, after it is disturbed.


Basic Aerodynamics



•To insure an airplane has good handling qualities in all flight regimes, we need it to be STABLE, MANEUVERABLE, and CONTROLLABLE•MANEUVERABLILITY is the characteristic that permits the pilot to easily move the airplane about its axes and to withstand the stress from these moves


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•To insure an airplane has good handling qualities in all flight regimes, we need it to be STABLE, MANEUVERABLE, and CONTROLLABLE•CONTROLLABILITY is the capability to respond to the pilot’s control inputs


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•Unfortunately, these characteristics are at odds with each other

•Increase in one leads to a decrease in another• = all airplane designs are compromises

•If make it too stable = it’s hard to control


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ControlsAircraft Stability•Stability: General Types

•2 TYPES OF STABILITY

•STATIC•The ability of an object to return to its equilibrium state after being disturbed

•DYNAMIC•The way the object moves after being disturbed


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•3 CONDITIONS OF STABILITY:

•POSITIVE•The disruption of an object gets less over time

•NEGATIVE •The disruption gets greater over time

•NUETRAL•The disruption neither increases or decreases over time


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•Positive Stability:

•The tendency to return to the original equilibrium•Example: Ball in a trough•Generally desirable in an airplane but does decrease maneuverability


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•Negative Stability:

•The tendency to move away from the equilibrium•Example: Ball on a hill•Undesirable in an airplane


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•Nuetral Stability:

•The tendency for the correcting forces to neither increase or decrease over time•Example: Ball on a flat surface•Somewhat OK in an airplane


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ControlsAircraft Stability•Stability: About the Aircraft Axes

•Since we have 3 main axes of an aircraft, we also have 3 main types of Stability:

•LONGITUDINAL•LATERAL•DIRECTIONAL


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•LONGITUDINAL STABILITY

•Is the ability of an aircraft to remain stable ABOUT (OR AROUND) THE LATERAL AXIS•Is PITCH STABILITY•Or keeping the Longitudinal Axis stable


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•LONGITUDINAL STABILITY•Since the CENTER OF PRESSURE (or CENTER OF LIFT) moves with Angle of Attack changes


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•LONGITUDINAL STABILITY•We need to be sure the Center of Gravity doesn’t get behind the Center of Pressure or severe flight problems will occur (such as can’t lower the nose)


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•LONGITUDINAL STABILITY•Airplanes are designed so the Center of Pressure or Lift is behind of the Center of Gravity = a downward pitching moment on the nose of the aircraft at all times


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•LONGITUDINAL STABILITY•This, coupled with the downward TAIL LOAD created by the HORIZONTAL STABILIZER, create a balanced set of conditions to keep the Longitudinal Axis stable


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•LONGITUDINAL STABILITY•If the aircraft gets disturbed so the nose goes up, the Horizontal Stabilizer’s Angle of Attack is decreased and creates less Down Load to compensate

•Hor. Stabs. are usually symmetrical airfoils


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•LONGITUDINAL STABILITY•If the aircraft gets disturbed so the nose goes down, the Horizontal Stabilizer’s Angle of Attack is increased and creates more Down Load to compensate•This is Positive Longitudinal Stability


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•LONGITUDINAL STABILITY•The Horizontal Stabilizer will be installed at some particular Angle of Incidence so it can do its job correctly

•This may be negative, positive, or zero


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•LATERAL STABILITY

•Is the ability of an aircraft to remain stable ABOUT (OR AROUND) THE LONGITUDINAL AXIS•Is ROLL STABILITY•Or keeping the Lateral Axis stable


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•LATERAL STABILITY•Provided mostly by wing DIHEDRAL

•This is the upward angle between the wing and the Lateral Axis


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•As an airplane is upset so a wing drops, it starts to SIDESLIP toward the low wing


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•This slipping motion plus the downward movement of the wing add downward vectors to the Angle of Attack production and lead to an increase in on the lower wing = more lift on that wing


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•On the wing moving up, the upward vector reduces the = less lift


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•These two changes to lift create a rolling force in the direction to restore the wings to level = Positive Lateral Stability

Author: Harry L. Whitehead 25

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•LATERAL STABILITY•A HIGH-WING Airplane will not need as much Dihedral as a LOW-WING Airplane •since the Center of Gravity is below the Center of Lift it tends to right itself naturally (it’s inherently more Laterally Stable)


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•DIRECTIONAL STABILITY

•Is the ability of an aircraft to remain stable ABOUT (OR AROUND) THE VERTICAL AXIS•Is YAW STABILITY•Or keeping the Vertical Axis stable


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•DIRECTIONAL STABILITY•Provided by the VERTICAL STABILIZER and Fuselage

•To be Directionally Stable, an aircraft must have more surface area behind the CG than in front so it acts like a Weather Vane


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•DIRECTIONAL STABILITY•Provided by the VERTICAL STABILIZER and Fuselage

•When the aircraft yaws (= SIDESLIP), the Vert. Stab. creates lift in the restoring direction and the sides of the fuselage offer a surface for the wind to push against


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•DIRECTIONAL STABILITY•Is also improved by SWEEPBACK of the wings


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•DIRECTIONAL STABILITY•Is also improved by SWEEPBACK of the wings

•When yawing (SIDESLIP), the wing which is moving forward has a larger effective wing area = more drag to push it back


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•DIRECTIONAL STABILITY•But sweepback can cause a small problem: Dutch Roll

•If the aircraft’s wing drops it will tend to yaw into the low wing and the dihedral and sweepback will combine to return the wings to level quickly


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•As the low wing moves up the Lateral Stability will return the aircraft to straight flight = the low wing will be moving faster than the high wing = more lift


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•= that wing will now rise and the process will repeat in the opposite direction


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•The resulting low level oscillation (“Dutch Roll”) doesn’t affect aircraft flight safety but is uncomfortable for passengers


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•Aircraft susceptible to this usually have YAW DAMPERS connected to the rudder controls to automatically apply corrective rudder action


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ControlsLarge Aircraft Controls•Large aircraft, like small, control the aircraft about the same 3 axes: Lateral, Longitudinal, and Vertical•Major differences:

•More control surfaces •Hydraulic actuated

•Power-boosted•Hydraulic cylinder in parallel with control rods•Pilot moves surface and valve to actuate hydraulic cylinder to help


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•Power-boosted•Boosting is typically about 14:1 ratio•Disadvantage: in transonic speed range shock waves form on controls and cause buffeting which is fed back into cockpit


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•Irreversibles•Used to keep buffet from reaching cockpit•Hyd. Cylinders in series with control rods•Also need “feedback” system to give pilot the feel of the controls


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ControlsLarge Aircraft Controls

•Example: Boeing 727


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•Uses Irreversible system with 2 separate hydraulic systems, Standby system, and manual backup of Primary Controls (servo tabs)


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•Example: Boeing 727•Primary Controls: Roll

•Ailerons and Spoilers•4 ailerons and 14 spoilers


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•Inboard ailerons and 10 flight spoilers do high speed flight with outboard ailerons locked in neutral position


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•When trailing edge flaps are deployed, outboard ailerons unlocked = all ailerons and flight spoilers work


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•Example: Boeing 727•Primary Controls: Pitch

•Elevators for normal pitch action•Movable Horizontal Stab. for trim action


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•Example: Boeing 727•Primary Controls: Yaw

•2 independent rudders with anti-balance (anti-servo) tabs


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•Example: Boeing 727•Primary Controls: Yaw

•Also receives input from the Yaw Dampers to counteract Dutch Roll


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•Example: Boeing 727•Auxiliary Lift Devices: Trailing Edge Flaps

•Triple-slotted Fowler Flaps•Take-off = only back, Landing = back and down


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•Example: Boeing 727•Auxiliary Lift Devices: Leading Edge Flaps

•Krueger-type•Increase area and camber


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•Example: Boeing 727•Auxiliary Lift Devices: Leading Edge Slats

•Increase camber•Inboard flaps stall first = retain aileron control


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•Example: Boeing 727•Secondary Controls (Tabs):

•Ailerons have Balance Tabs which also act as Trim Tabs


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•Elevators have Servo Tabs which also act as Trim Tabs


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•Rudders have Anti-balance (anti-servo) Tabs which also act as Trim Tabs


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•These Tabs also serve as manual backups in case of total hydraulic failure


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•Example: Lockheed L-1011


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•Example: Airbus A320


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Controls

basic aerodynamics ii stability large

Business

controllable stability

basic aerodynamics

conditions of stability

airplane negative stability

types of stability static

airfoil lift

atmosphere physics

flight regimes